Trailer videos don't stop when Steam window is closed

[SolarLune]

Your system information

• Steam client version (build number or date): Nov. 8 2017
• Distribution (e.g. Ubuntu): Linux Mint KDE
• Opted into Steam client beta?: Yes
• Have you checked for system updates?: Yes

Please describe your issue in as much detail as possible:

When I was watching a trailer video on a game's page, I expected the trailer to stop being played when I close the Steam window. The video continued, though (which is a bug).

Steps for reproducing this issue:

1. Play a trailer video on a game's page.
2. Close Steam's main window.
3. The video will keep playing (and so the audio will also keep playing), until you open Steam back up to another page.

[mbcraft]

Let me know if you need some suggestions on how to fix this issue. Maybe just fixing this one is enough and the other problems i pointed out in my issue tracking won't happen if this issue is fixed ...